You're absolutely right that there are external factors that can affect people, however they're certainly not insurmountable, not even especially technically difficult to surmount. However, there's a huge gap in nutritional education for many of these people living at or below the poverty line.

All of these factors that you describe can be compensated for by consuming fewer calories to match the fewer that the body is burning. It's all very simple, but it's not easy.

I think a large part of the problem that people have with food is that it's become more than just food to most. People don't eat for nutrition or to sustain themselves, they eat because they like a particular type of food, or because they want to go out to lunch with coworkers, or grab a bite and a drink with friends after work.

It creates an unhealthy association between food and socializing where people consume for reasons that our bodies were never designed for.

Outside of basic education during formative years though, I don't really have an answer to the issue. I do know that people have control over their weight in all but the most infrequent of circumstances (Prader-Wiley comes to mind), and denying that gives the impression that people don't have the power to change themselves. If they don't choose to change themselves, then that's fine, but don't hide it under the guise of not being able to. People need to know what the consequences of their obesity are, and how controllable it all is. If you look at trends of obesity in America over the past 50 years and compare it with health consequences, you can draw a pretty scary picture. I don't believe that encouraging youths today to continue with known, unhealthy lifestyles is conscionable, and the only solution I see is a concerted effort at educating young people to healthy lifestyles at an early age before they can develop the bad habits that are so difficult to break.